Generally, a light beam scanning device which is used for image forming apparatuses, such as a laser printer, a digital copier, and a multifunction printer, is adapted to deflect a light flux emitted from a laser light source by a deflecting optical system, such as a polygon minor and to form an image as a light spot on a scanned surface by a scanning imaging optical system.
As the laser light source, a semiconductor laser and the like are generally used. A divergent light flux emitted from the laser light source is converted in an almost parallel light beam by a collimator lens and the contour of the parallel light beam is shaped by an aperture. The light beam whose contour is shaped, is deflected by a deflecting optical systems, such as a polygon mirror rotating at a constant angular velocity, and enters a scanning imaging optical system. The scanning imaging optical system is required to have the fθ characteristic to make a light beam deflected with a constant angular velocity to scan at an equal distance velocity on a scanned surface arranged with a predetermined gap distance, and to have a field curvature corrected in good order so as to form a minute optical spot over all a scanning region.
In the case that a polygon minor is used for the deflecting optical system, since the polygon mirror has working errors on minor surfaces, an vibrations on a rotating shaft, and so on, many imaging lens systems are provided with an inclination correcting function to correct a deviation in a scanning position in a sub-scanning direction (a vertical scanning direction, a direction vertical to a main scanning direction). For this reason, an imaging lens system is made into an anamorphic lens system with which an imaging characteristic in a main scanning direction differs from that in a sub-scanning direction.
Further, conventionally, the above-mentioned scanning imaging optical system is manufactured with a glass material. However, since the processing of a glass lens is difficult and becomes a high cost, it is required in recent years to manufacture a lens by a plastic material with which the lens can be manufactured at a low cost can and is shaped in a free form so as to rectify an aberration.
Furthermore, conventionally, a semiconductor laser used as a light source is generally an infrared laser (a wavelength of about 780 nm) or a red laser (a wavelength of about 650 nm). However, these days, in response to a request to make a resolution more high, a light beam scanning device has been developed so as to obtain a minute spot configuration by the use of a short wavelength light source with, a wavelength of 500 nm or less. Moreover, recently, in response to a request to make a device in a compact size, it is required to make a polygon mirror and a plastic lens in a small size by the use of a short wavelength light source and by making the F-number of a scanning imaging optical system larger so as to make a beam diameter smaller.
However, in many optical materials used as a plastic lens, as a wavelength becomes shorter, the transmittance may become smaller due to internal absorption. In particular, in the case that a short wavelength light source with a wavelength of 500 μm or less is used, the transmittance reduces largely due to internal absorption. Moreover, it turns out that when a plastic lens is irradiated with a light beam having a short wavelength for a long time, there is a problem that the plastic lens causes white turbidity (or cloudy) so as to reduce more its transmittance.
For such a problem, as a light beam scanning device employing a light source with a wavelength of 500 nm or less, a light beam scanning device is proposed such that at least one lens of a scanning imaging optical system is made as a plastic lens (refer to Patent Document 1).
Furthermore, another light beam scanning device is proposed such that a light source with a wavelength of 450 nm or less is used, an optical system between the light source and a deflecting optical system is structured with optical elements made of glass, and a scanning imaging optical system is structured with optical elements made of plastic (refer to Patent Document 2). In this light beam scanning device, the optical elements through which a light flux passes continuously are made as optical elements made of glass, whereas the optical elements through which a light flux does not pass continuously are made as optical elements made of plastic, whereby the light beam scanning device is provided at a low cost    Patent Document 1: U.S. Pat. No. 3,478,819 official report    Patent Document 2: Japanese Unexamined Patent Publication No. 2006-313268 official report